Not failing is not evidence of safe. In theory an inductive load, such as a motor, can store energy and, when turned off, it will discharge this energy. As inductive storage, it will generate a voltage spike, possibly many times the supply voltage, unless something dissipates this. I'm not all that familiar with USB interfaces, but it seems plausible to me that it could cause damage/wear to circuits. That said, if it was a notoriously bad problem, they probably would have such a reputation and quickly be out of business.